This invention relates to multi-mode operation of a standing wave linear accelerator for producing a diagnostic beam or a therapeutic beam, or both.
Radiation therapy involves delivering a high, curative dose of radiation to a tumor, while minimizing the dose delivered to surrounding healthy tissues and adjacent healthy organs. Therapeutic radiation doses may be supplied by a charged particle accelerator that is configured to generate a high-energy (e.g., several MeV) electron beam. The electron beam may be applied directly to one or more therapy sites on a patient, or it may be used to generate a photon (e.g., X-ray) beam, which is applied to the patient. An x-ray tube also may supply therapeutic photon radiation doses to a patient by directing a beam of electrons from a cathode to an anode formed from an x-ray generating material composition. The shape of the radiation beam at the therapy site may be controlled by discrete collimators of various shapes and sizes or by multiple leaves (or finger projections) of a multi-leaf collimator that are positioned to block selected portions of the radiation beam. The multiple leaves may be programmed to contain the radiation beam within the boundaries of the therapy site and, thereby, prevent healthy tissues and organs located beyond the boundaries of the therapy site from being exposed to the radiation beam.
X-ray bremsstrahlung radiation typically is produced by directing a charged particle beam (e.g., an electron beam) onto a solid target. X-rays are produced from the interaction between fast moving electrons and the atomic structure of the target. The intensity of x-ray radiation produced is a function of the atomic number of the x-ray generating material. In general, materials with a relatively high atomic number (i.e., so-called xe2x80x9chigh Zxe2x80x9d materials) are more efficient producers of x-ray radiation than materials having relatively low atomic numbers (i.e., xe2x80x9clow Zxe2x80x9d materials). However, many high Z materials have low melting points, making them generally unsuitable for use in an x-ray target assembly where a significant quantity of heat typically is generated by the x-ray generation process. Many low Z materials have good heat-handling characteristics, but are less efficient producers of x-ray radiation. Tungsten typically is used as an x-ray generating material because it has a relatively high atomic number (Z=74) and a relatively high melting point (3370xc2x0 C.).
The bremsstrahlung process produces x-rays within a broad, relatively uniform energy spectrum. Subsequent transmission of x-rays through an x-ray target material allows different x-ray energies to be absorbed preferentially. The high-Z targets typically used for multi-MeV radiation therapy systems produce virtually no low energy x-rays (below around 100 keV). The resultant high energy x-rays (mostly above 1 MeV) are very penetrating, a feature that is ideal for therapeutic treatment. In fact, in treatment applications, it is desirable not to have a significant amount of low energy x-rays in the treatment beam, as low-energy beams tend to cause surface burns at the high doses needed for therapy.
Before and/or after a dose of therapeutic radiation is delivered to a patient, a diagnostic x-ray image of the area to be treated typically is desired for verification and archiving purposes. The x-ray energies used for therapeutic treatment, however, typically are too high to provide high quality diagnostic images because high-energy therapeutic beams tend to pass through bone and tissue with little attenuation. As a result, very little structural contrast is captured in such images. In general, the x-ray energies that are useful for diagnostic imaging are around 100 keV and lower. High-Z targets produce virtually no x-rays in this diagnostic range. Low-Z targets (e.g., targets with atomic numbers of 30 or lower, such as aluminum, beryllium, carbon, and aluminum oxide targets), on the other hand, produce x-ray spectra that contain a fraction of low-energy x-rays that are in the 100 keV range and, therefore, are suitable for diagnostic imaging applications. See, for example, O. Z. Ostapiak et al., xe2x80x9cMegavoltage imaging with low Z targets: implementation and characterization of an investigational system,xe2x80x9d Med. Phys., 25 (10), 1910-1918 (October 1998).
In addition to changing x-ray targets, other methods of varying the output energy of a radiation system have been proposed.
For example, U.S. Pat. No. 4,024,426 discloses a standing-wave linear accelerator that includes a plurality of electromagnetically decoupled side-cavity coupled accelerating substructures such that adjacent accelerating cavities are capable of supporting standing waves of different phases. The phase relationship between substructures may be adjusted to vary the beam energy.
U.S. Pat. No. 4,286,192 discloses a variable energy standing wave guide linear accelerator in which the radio frequency mode in a coupling cavity may be changed to reverse the field direction in part of the accelerator. In particular, the mode of a side cavity is adjusted so that the phase introduced between adjacent main cavities is changed from X to zero radians. The field reversal acts to decelerate the beam in that part of the accelerator.
U.S. Pat. No. 4,629,938 describes a standing wave linear accelerator with a side cavity that may be detuned to change the normal fixed phase shift of the main cavities adjacent to the detuned side cavity, and to decrease the electric field strength in cavities downstream from the detuned side cavity.
Still other variable energy standing wave linear accelerator schemes have been proposed.
The invention features systems and methods for multi-mode operation of a standing wave linear accelerator to produce charged particle beams with different output energies. The resulting charged particle beams may be used to produce a relatively high energy therapeutic beam or a relatively low energy diagnostic beam, or both.
In one aspect, the invention features a method of generating charged particle beams of different output energy. In accordance with this method, a standing wave linear accelerator is operated in a first resonance mode to produce a first charged particle beam characterized by a first output energy, and the standing wave linear accelerator in a second resonance mode to produce a second charged particle beam characterized by a second output energy different from the first output energy.
Embodiments in accordance with this aspect of the invention may include one or more of the following features.
The first output energy preferably is suitable for performing diagnostic imaging of a patient. For example, the first output energy may be less than about 1,000-1,500 keV.
The second output energy preferably is suitable for performing therapeutic treatment of a patient. For example, the second output energy may be between about 4 MeV and about 24 MeV.
The standing wave linear accelerator preferably is operated in a non-xcfx80/2 resonance mode to produce the first charged particle beam, and the standing wave linear accelerator preferably is operated in a xcfx80/2 resonance mode to produce the second charged particle beam.
One or both of the first and second charged particle beams may be intercepted with an energy filter or an energy absorber.
In another aspect, the invention features a method of performing diagnostic imaging of a patient. In accordance with this method, a standing wave linear accelerator is operated in a non-xcfx80/2 resonance mode to produce a charged particle beam. A diagnostic beam is produced from the charged particle beam. The patient is imaged based upon passage of the diagnostic beam through the patient.
In another aspect, the invention features a system for generating charged particle beams of different output energy that includes a standing wave linear accelerator, and a controller configured to implement the above-described methods.
Among the advantages of the invention are the following.
The invention provides a scheme in accordance with which a linear accelerator may be operated in two or more resonance (or standing wave) modes to produce charged particle beams over a wide range of output energies so that diagnostic imaging and therapeutic treatment may be performed on a patient using the same device. In this way, the patient may be diagnosed and treated, and the results of the treatment may be verified and documented, without moving the patient. This feature reduces alignment problems that otherwise might arise from movement of the patient between diagnostic and therapeutic exposure machines. In addition, this feature reduces the overall treatment time, thereby reducing patient discomfort.
Other features and advantages of the invention will become apparent from the following description, including the drawings and the claims.